Molten salt environment creep testing extensometry system

ABSTRACT

Disclosed herein are systems, devices and methods for creep testing selected materials within a high-temperature molten salt environment. Exemplary creep testing systems include a load train for holding a test specimen under a load within a heated inert gas vessel. An extensometry system can be included to measure elongation of the test specimen while under load. The extensometry system can include fixed members and axially translating member that move along with the elongation of the test specimen, and the system can include a sensor to measure the relative axial motion between such components to measure elongation of the test specimen over time. The test specimen can include a cylindrical gage portion having an internal void filled with a molten salt during creep testing to simulate the corrosive effect of the molten salt on the specimen material during testing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/460,274 filed on Feb. 17, 2017, which is incorporatedby referenced herein in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in this invention.

FIELD

This application is related to testing material properties forapplications in high-temperature molten salt environments, in particularby using an extensometry system to measure properties of a test specimenwhile exposed to a high-temperature molten salt environment.

BACKGROUND

Molten salts enable efficient, low-pressure heat transfer at hightemperatures. No standardly available heat transfer fluids exist with anoperational temperature above 600° C. Molten salt mediums enable higheroperating temperatures and thus higher thermodynamic efficiency in hightemperature nuclear reactors, solar thermal power systems, and similarsystems for aluminum manufacturing industry, thermal energy storage, andhigh temperature heat transfer for process heat applications. Comparedto water-cooled reactors, for example, reactors using molten salt mayoperate at higher temperatures and thus provide higher thermodynamicefficiency. However, molten salts can be corrosive to the structuralmaterials, necessitating careful material selection and salt chemistrycontrol during operation. Thus, there is a need for systems and methodsfor testing the ability of heat transfer structural materials towithstand the high temperature, potentially corrosive conditions ofmolten salts.

SUMMARY

Disclosed herein are systems, devices and methods related to creeptesting, stress-rupture testing, relaxation testing, and non-compressivefatigue testing and creep-fatigue testing of selected materials within ahigh-temperature molten salt environment.

Some disclosed systems, devices and methods are for creep testingselected materials within a high-temperature molten salt environment.Exemplary creep testing systems include a load train for holding a testspecimen under a load within a heated inert gas vessel. An extensometrysystem can be included to measure elongation of the test specimen whileunder load. The extensometry system can include fixed members andaxially translating member that move along with the elongation of thetest specimen, and the system can include a sensor to measure therelative axial motion between such components to measure elongation ofthe test specimen over time. The test specimen can include a cylindricalgage portion having an internal void filled with a molten salt duringcreep testing to simulate the corrosive effect of the molten salt on thespecimen material during testing.

An exemplary system for testing of selected materials in ahigh-temperature molten salt environment includes a vertically orientedload train that holds a test specimen in-line and is configured to applya tension load to the test specimen. The load train is positionedpartially within an enclosed vessel that is positioned within orincludes a heating mechanism. The vessel can include one or more gasports for maintaining a controlled inert gas environment within thevessel around the load train, one or more sensor ports, and/oradditional access ports for liquid cooling lines and other purposes. Theload train can include a first pull rod positioned at least partiallywithin the vessel and extending downwardly from or through the upper endof the vessel, and a second pull rod positioned within the vessel belowthe first pull rod. The first pull rod has a first specimen grip at alower end of the first pull rod and the second pull rod has a secondspecimen grip at an upper end of the second pull rod. The first andsecond specimen grips are adapted to grip an upper end and a lower endof a test specimen that is being tested. The specimen can include agenerally cylindrical tubular gage portion that contains a salt thatmelts when the testing system is heated to testing temperatures andcreates a salt-specimen interface within the gage portion.

Below the heating mechanism, in certain embodiments, the load trainincludes a thermal break that couples a lower end of the second pull rodand an upper end of a third pull rod, while minimizing heat conduction.In some embodiments, the thermal break can comprise a rigid fixturecoupled to the lower end of the second pull rod and a thermallyinsulating rigid spacer, such as a ceramic disk, supported by thefixture below the second pull rod. The upper end of the third pull rodis supported by the spacer (which is in compression) but is spaced belowthe lower end of the second pull rod and spaced within the thermal breakfixture, such that the third pull rod is thermally decoupled from thesecond pull rod to reduce heat conduction down the load train into thethird pull rod.

This is advantageous because it allows for a load cell to be placed inthe third pull rod to measure the load applied to the test specimen. Theload cell can be shielded from the high-temperatures higher up the loadtrain, which would otherwise damage the load cell. This allows the loadcell to be placed within the vessel instead of being located below thevessel. The third pull rod can have a lower end below the load cell thatextends through a lower end of the vessel and is adapted to be coupledto a loading source for applying a load to the test specimen via thesecond and third pull rods and the thermal break. Friction can occurbetween the third pull rod and the lower end of the vessel which candistort the amount of load applied to the test specimen compared to theactual load applied outside of the vessel. Thus, locating the load cellwithin the vessel eliminates such distortion and provides a moreaccurate load measurement.

In some embodiments, a gap is provided between the third pull rod andthe lower opening of the vessel to ensure a minimization or eliminationof friction. Additional low-friction bearings or rings can be includedat the interface to keep the gap consistent and allow load transfer withminimal interference by friction. A gas bubbler and a flowmeter areprovided to an inlet port to monitor and control the inert gas pressureinside the enclosed vessel such that the inert protective gas fromwithin the vessel is allowed to exit the vessel through the gap duringtesting to prevent air ingress.

In some embodiments, no gap is provided between the third pull rod andthe lower opening of the vessel, and a bellows-type seal is used toprevent air ingress and allow motion of the third pull rod as the testspecimen elongates during testing while the load cell signal is beingfed into a closed-loop system to control the applied load so that theforce from the bellows-type seal does not affect the applied load on thetest specimen.

The system can be configured to apply a tension load to the gage portionof the test specimen while the test specimen is maintained at usetemperatures wherein the salt is molten, such as at a temperature thatis at least 100° C. greater than the melting temperature of the salt,such as greater than 700° C. for example. Exemplary salts can comprise2⁷LiF—BeF₂ and the low melt point eutectic of KF—ZrF₄.

An exemplary test specimen disclosed herein includes a first end portionhaving a first engagement portion for connecting to a testing system, asecond end portion having a second engagement portion for connecting tothe testing system, a narrowed gage portion between the first and secondend portions, and an inner void extending through the first end portionand through the gage portion. The gage portion has a substantiallycylindrical outer surface defining an outer diameter and the inner voidis substantially cylindrical within the gage portion such that the gageportion has a substantially cylindrical inner surface defining an innerdiameter and the gage portion has a substantially constant wallthickness between the inner diameter and the outer diameter. The innervoid is configured to receive a solid salt ingot or powdered salt suchthat when the test specimen is subjected to high temperatures, the saltmelts to form molten salt that completely fills the portion of the voidthat is within the gage portion.

In some embodiments, a ratio Ai/V of the gage portion is at least about20, or from about 20 to about 32, wherein Ai is the inner surface areaof the gage portion and V is the volume of test specimen material (e.g.,a metal alloy) in the tubular gage portion between the inner surface ofthe gage portion and the outer surface of the gage portion, in units ofsquare inches divided by cubic inches.

In some embodiments, a basin ring is attached around the test specimenbelow the gage portion and configured to catch the molten salt thatescapes from inside the test specimen when the test specimen rupturesand leaks during testing.

Exemplary methods of stress rupture testing and/or creep testing of amaterial in a high-temperature molten salt environment can includecreating a solid salt ingot (or salt powder), placing the salt ingot (orsalt powder) within an inner void of a test specimen and sealing thevoid closed, mounting the test specimen with the salt ingot enclosed ina load train within a vessel of a testing system, filling the vesselwith an inert gas, heating the test specimen while mounted in the loadtrain within the vessel filled with inert gas such that the salt ingotmelts within the void and the resulting molten salt contacts an entireinner surface of a gage portion of the test specimen, applying andsustaining a load on the gage portion of the test specimen while thetest specimen is mounted in the load train within the vessel filled withinert gas and the salt is molten, and measuring the applied load, timeelapsed, gage portion elongation, and/or other parameters until the gageportion of the test specimen fails. The inert gas can be continuouslyfed into the vessel while the load is applied and a small quantity ofinert gas is allowed to escape from the vessel through the gap betweenthe load train and a lower end of the vessel to ensure no air ingressoccurs.

Exemplary methods for forming a salt ingot for use in the testing systemcan include: placing a mold within a vacuum chamber, creating an inertgas environment within the vacuum chamber around the mold, heating themold within the vacuum chamber in the inert environment to removeimpurities from the mold, placing a salt into the mold and closing themold in an inert environment, heating the mold to remove impurities suchas moisture and oxygen, melting the salt to remove voids from the salt,cooling the mold to solidify the salt into a salt ingot with impuritiesremoved, and transferring the salt ingot from the mold into a testspecimen in an inert environment. Creating an inert gas environmentwithin the vacuum chamber can comprise drawing a vacuum on the vacuumchamber and feeding an inert gas into the vacuum chamber to flush outambient air from the vacuum chamber. The method can further comprise:placing a funnel and a mold housing within a vacuum chamber along withthe mold; heating the mold, funnel, and mold housing within the vacuumchamber in the inert environment to remove impurities from the mold,funnel, and mold housing; transporting the mold and funnel sealed withinthe mold housing from the vacuum chamber to a salt-filling chamberhaving an inert environment; using the funnel to place purified saltinto the mold in the salt-filling chamber; and/or transporting thesalt-filled mold sealed within the mold housing from the salt-fillingchamber to a vacuum chamber.

The foregoing and other objects, features, and advantages of thedisclosed technology will become more apparent from the followingdetailed description, which proceeds with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cross-sectional side view of an exemplary testingsystem disclosed herein.

FIG. 2 shows an upper portion of the system of FIG. 1.

FIG. 3 shows an intermediate portion of the system of FIG. 1.

FIG. 4 shows a lower portion of the system of FIG. 1.

FIG. 5 is a perspective view of an exemplary test specimen used in thesystem of FIG. 1.

FIG. 6 is a side view of the test specimen of FIG. 5.

FIG. 7 shows various exemplary components of a salt ingot preparationsystem.

FIG. 8 shows an exemplary salt ingot preparation system.

FIG. 9 shows an upper portion of another exemplary testing system.

FIG. 10 shows an intermediate portion of the system of FIG. 9.

FIGS. 11 and 12 show two orthogonal views of a lower portion of thesystem of FIG. 9.

DETAILED DESCRIPTION

Disclosed herein are devices, systems and methods for testing theability of materials to withstand applied stress, and/or to measurecreep or strain of the material under load, while being exposed tohigh-temperature and corrosive environments that simulate conditionswithin a molten salt heat transfer/heat storage system, such as withinmolten salt nuclear reactors or solar thermal power systems. Forexample, large solar panel arrays can generate a great amount of heatduring the day, and that heat can be stored using a molten salt heatstorage system during the day and then utilized to continue generatingpower during the night when the solar panels are not producing power.

In an exemplary testing procedure, a test specimen is formed of amaterial of interest (e.g., a metal alloy) and the test specimen isplaced under load while in contact with a molten salt (also referred toas molten salt) and at high temperatures. The test specimen's ability towithstand the applied load under such conditions is measured, such as tocharacterize how well the material of interest is likely to perform ifused in a molten salt heat transfer system.

The disclosed testing systems can provide one or more advantages thatallow for accurate testing of material performance in the harshconditions of a molten salt nuclear reactor or molten salt heatexchanger. For example, some disclosed testing systems are able tofunction at very high temperatures, such as at least 700° C., a range offrom 700° C. to 1000° C., or even greater than 1000° C., which cansimulate the operating temperatures of modern and future molten saltreactors. For example, in some embodiments, the testing system can heatthe test specimen to a temperature that is at least 100° C. greater thanthe melting temperature of the salt. Also, certain embodiments of thedisclosed testing systems are able to safely contain and apply corrosivesalts, such as 2⁷LiF—BeF₂, KF—ZrF₄, other fluoride or chloridecontaining salts, and the like, at such high temperatures, whileminimizing contamination of the salts, in solid and liquid phases,before and during the preparation and testing procedures. In addition,certain embodiments of the disclosed testing systems are configured toprovide enough molten salt in contact with the test specimen material toadequately test the materials resistance to the corrosive effects of themolten salt while under stress, while minimizing exposure of the testmaterial to undesired influences, such as oxygen and moisture fromatmospheric air. Further, the load train and specimen holders of certainof the disclosed testing systems are constructed to provide highstress-rupture strength at such very high temperatures, and aredesirably protected from the corrosive molten salts that are in contactwith the test specimen during testing. Furthermore, disclosed systemscan include mechanisms to accurately control and measure the loads thatare applied to the test specimen during testing and/or the elongation orcreep of the test specimen while under load, while being protected fromthe high-temperature, corrosive testing environment. Many otheradvantages and benefits can also be provided by the disclosed systemsand methods.

FIG. 1 shows an exemplary testing system 2 for testing a test specimen 4in an environment simulating a molten salt nuclear reactor or moltensalt heat exchanger. FIGS. 2-4 show enlarged views of different portionsof the system 2, and FIGS. 5 and 6 show detailed views of the testspecimen 4.

As shown in FIG. 1, the test specimen 4 is held in a substantiallyvertical orientation in a load train of the testing system 2 andpositioned within an elongated vessel 6. In other embodiments, the loadtrain can be oriented non-vertically, such as horizontally or at otherangles from vertical. The illustrated embodiment is oriented verticallyto utilize gravitational forces to apply constant tension to the testspecimen and ensure that the molten salt fills the gage portion of thespecimen during testing. However, in other embodiments other mechanisms,such as electromechanical or servo hydraulic system, can be used toapply tension to the test specimen while the specimen is at a verticalor non-vertical orientation, while maintaining sufficient surfacecontact of the molten salt against the specimen material. Theclosed-loop electromechanical or closed-loop servo hydraulic systemallows application of varying loading to test specimen for fatigue andcreep-fatigue properties of the selected material.

The vessel 6 is used to provide a controlled environment around the testspecimen and the load train during testing. For example, the vessel 6can be filled with an inert gas, such as argon, during testing toprotect the load train components and the test specimen from oxidizingor otherwise reacting to the ambient environment in the vessel at hightemperatures. For example, in some embodiments, load train componentscan comprise titanium-zirconium-molybdenum alloy (TZM), which can besusceptible to reaction with oxygen and other elements in atmosphericair at high testing temperatures. The vessel 6 can also be constructedof a non-reactive material, such as alumina. The vessel 6 can besupported independently from the load train, such as by mountingbrackets 26 and 27. The system 2 also includes a heating mechanism, suchas the furnace 8, adjacent to the vessel 6 for maintaining the testspecimen and salts at a desired temperature during testing.

The load train can comprise an aligned series of components that holdthe test specimen 4 within the testing system 2 and apply desired loadsto the test specimen during testing. The load train can comprise uppercomponents that couple the test specimen 4 to a stationary uppermounting location and lower components that couple the test specimen toa loading source (not shown) below the system 2. When a load is appliedby the load train during testing, a gage portion 74 of the test specimen4 (see FIGS. 5 and 6), which is exposed to the molten salt, is stressedto determine the test specimen material's ability to withstand theenvironmental rigors of a molten salt nuclear reactor. The data are thenused to characterize the material being tested.

As shown in the illustrated embodiment, for example, the upper part ofthe load train (see FIGS. 2 and 3) can include couplings 10, 14 and pullrods 12, 16 that couple an upper end of the test specimen 4 to astationary upper support (not shown) via a threaded upper end 22 of thepull rod 16. The upper portion of the load train extends through anopening at the upper end 18 of the vessel 6, which can be sealed, suchas with a bellows-type seal 20, to prevent inert gas from escaping oratmospheric gas from entering the vessel.

The system 2 can include thermal shields 30, 36 mounted on the loadtrain above and below the test specimen 4. The shields 30, 36 reduceconvection and radiation heat loss from the testing zone where thespecimen gage section locates to facilitate maintaining a uniformtemperature gradient within the testing zone and minimizing heat outsidethe testing zone. The shields 30, 36 can be spaced slightly from theinner walls of the vessel 6 to avoid frictional forces on the load trainthat can disrupt the testing procedure.

As shown in FIGS. 3 and 4, the lower part of the load train couples thelower end of the test specimen to the loading source. A lower end of theload train extends through a seal assembly 56 at the lower end of thelower chamber 50 of the vessel 6 and includes a threaded lower end 66that is configured to attach to the loading source below the testingsystem 2. The loading source can be a dead weight hanging from the loadtrain, an electromechanical or servo hydraulic system, or other loadingmechanisms known in the art. The lower part of the load train caninclude a thermal break between the components that are coupled to thelower end of the test specimen 4 within the furnace 8 (e.g., coupling 32and pull rod 34), and the lower components of the load train (e.g.,lower pull rod 44) that are coupled to the load source. The thermalbreak significantly limits heat conduction downwardly through the loadtrain, which protects temperature sensitive components below the thermalbreak.

For example, a load cell 54 that is vulnerable to high temperatures canbe attached to, or be inserted between upper and lower portions of, thelower pull rod 44 below the thermal break for measuring the amount ofload applied to the test specimen. The load cell 54 can be electricallycoupled to data collection systems outside the system 2 via connection55. Placement of the load cell 54 within the vessel 6 allows for moreaccurate measurement of the load that is applied to the test specimen 4as it accounts for frictional influence between the lower pull rod 44and the seal assembly 56 of the vessel. Placement of the load cell 54within the vessel 6 is enabled by use of the thermal break. The loadcell 54 can be threadably inserted between an upper portion of the lowerpull rod 44 and a lower portion of the lower pull rod, such that theload cell can measure the total amount of force that is transmittedalong the lower pull rod 44.

The thermal break (see FIG. 4) provides a break in the thermalconduction pathway downward through the load train. The pull rods (e.g.,34, 44) and couplings (e.g., 32) of the load train can comprise a metalmaterial that maintains high strength at very high testing temperatures,such as TZM. Unlike other common metals, TZM load train componentsremain very strong and provide sufficient strength to the specimen gripsand other load train connections during the high temperature conditionsduring testing. However, TZM and other high-temperature-resistant metalscan also be good thermal conductors. The thermal break includesthermally insulating material, such as ceramic materials, such that thepull rod 34 above the thermal break is thermally insulated from thelower pull rod 44 below the thermal break.

In the embodiment of FIG. 4, thermal break includes a thermal breakadaptor 38, a thermal break fixture 40, and an insulating spacer 42. Theadaptor 38 and fixture 40 can comprise metal materials that are strongin tension, like the TZM pull rods 34 and 44. The adaptor 38 can bethreadably attached to the lower end of the pull rod 34, and the fixture40 can be threadably attached around the adaptor 38. The insulatingspacer 42 can comprise a ceramic disk, for example, that rests on alower ledge 49 of the fixture 40 and supports an upper rim or head 46 ofthe pull rod 44. The pull rod 44 passes through a lower opening 48 inthe fixture 40 without contacting the fixture to prevent direct thermalconduction from the fixture 40 to the pull rod 44. The upper head 46 ofthe pull rod 44 is flared radially such that the upper head forms alower contact surface that rests on top of the spacer 42. The spacer 42can fit tightly around pull rod 44 under the head 46 to restrain thepull rod 44 from moving radially relative to the spacer, and the spacer42 can also fit tightly within the fixture 40 to restrain the spacerfrom moving radially relative to the fixture. A ceramic material of thespacer 42 can provide high strength in compression between the pull rodhead 46 and the lower ledge 49 of the fixture 40, while also providingthermal insulation. Furthermore, the upper surface of the pull rod head46 is spaced from the lower surfaces of the pull rod 34 and adapter 38,and spaced radially from the inner surfaces of the fixture 40 tominimize thermal conduction. The adapter 38 can be removed from thefixture 40 to provide access for inserting the spacer 42 and the pullrod 44 into the fixture. The fixture 40 can comprise a generallycylindrical or tubular configuration or can have other suitableconfigurations.

The vessel 6 can comprise a lower chamber 50 below the furnace, such asa cross-chamber having lateral access ports. The lower chamber caninclude a cooling system 52 and related ports 53 for cooling the lowerend of the load train, such as a fluid that flows through a coil wrappedaround the pull rod 44 below the thermal break. The load cell 54 can bepositioned below the coil of the cooling system 52 such that the loadcell is further protected from thermal damage. In addition to thethermal break and the cooling system 52, the various components of thetesting system 2 can include additional cooling systems to dissipateheat. For example, the vessel 6 can include fluid ports 29 (see FIG. 2)and the lower chamber 50 can also include fluid ports for circulating acooling fluid, such as water. In addition, the upper pull rod 16 andlower pull rod 44 can include fluid-cooled adapters 24 and 64,respectively, for circulating cooling fluid around and/or through thepull rods. In some embodiments, the pull rods can include one or morehollow passageways extending through the pull rods for circulatingcooling fluid.

As shown in FIG. 4, the lower end of the vessel includes a seal assembly56 that allows the pull rod 44 to pass through the lower end of thevessel with minimal frictional resistance to relative vertical motion. Asmall gap 57 is allowed between the pull rod 44 and the inner walls ofan opening passing through the seal assembly 56 such that the pull rod44 does not contact the inner walls of the seal assembly. A bushing 58and/or a ring 60 can be positioned around the pull rod 44 and can fitsecurely within a lower recess 62 of the seal assembly below the gap 57to keep the pull rod centered in the lower opening and maintain thesmall gap 57, while allowing the pull rod to slide vertically relativeto the vessel with minimal friction. The bushing 58 and/or ring 60 canbe made of a material that has a very low coefficient of friction withthe pull rod 44. The bushing 58 and/or ring 60 can include one or morevertical holes, such as drilled holes, to further facilitate the escapeof inert gas through the gap 57. In an alternative embodiment, sealassembly 56 is replaced by a bellows-type seal like 20 to allow the pullrod 44 to pass through the lower end of the vessel to eliminate airingress while allowing relative vertical motion.

The vessel 6 can include one or more process gas inlets, such as inlet68 in the lower end seal assembly 56 (FIG. 4) and/or an inlet in theupper end 18 of the vessel (not shown), which allow an inert processgas, such as argon, to be fed into the vessel. The process gas canescape through the gap 57 and/or through other outlets, such as an upperoutlet at the top end of the vessel, while additional process gas can befed into the vessel through one or more inlets, such as the inlet 68, tomaintain a positive flow out of the vessel 6, which inhibits unwantedexternal gases from entering the vessel. During set up of the system,atmospheric air captured within the vessel 6 can initially be purged outthrough an upper gas outlet near the top end of the vessel (since theprocess gas is heavier than air) by feeding in the process gas throughone or more inlets, such as lower inlet 68, for a sufficient period oftime. Other ports can also be provided in the vessel 6, such as ports 19(FIG. 2) for thermocouples that attach to the test specimen, a salt leakindicator attached to a basin ring 78 (FIG. 3), and ports for othersensors and wiring.

FIGS. 5 and 6 show an embodiment of a test specimen 4 and the basin ring78 that is attached around a lower threaded portion 82 of the testspecimen 4 for catching molten salt that escapes from the test specimenwhen it ruptures or leaks during testing.

The test specimen 4 includes an upper connector 70, a lower connector72, and a gage portion 74 between the upper and lower connectors. Theconnectors 70 and 72 are threaded to attach to upper and lower couplings10 and 32, respectively, of the load train. The couplings 10 and 32 arealso referred to as grips. The threaded upper and lower connectors 70,72 are sized such that they are stronger in tension than the gageportion 74, such that stress-rupture failure occurs first in the gageportion. The test specimen 4 includes an inner void 76 that extendsthrough the gage section and the upper connector 70. The void 76 is openat the upper end of the upper connector for loading the salt and thenclosed off prior to testing (e.g., immediately after loading the salt)to ensure containment of the molten salt during testing and to protectthe salt from contamination. The void 76 is configured to be weldedclosed, for example, to contain the salt in its corrosive liquid phaseduring testing at high temperatures.

The test specimen 4 can have an overall longitudinal length L₁ of about5.67 inches (144 mm) and the gage portion 74 can have a longitudinallength L₂ of about 2.25 inches (57.2 mm), though the lengths L₁ and L₂can vary in other embodiments. The gage portion 74 can be substantiallycylindrical. The gage portion 74 can have a nominal outer diameter OD ofabout 0.50 inches (12.7 mm), while the OD can vary in other embodiments.The inner diameter ID of the gage portion 74 (e.g., the diameter of theinner void 76) and the radial wall thickness T of the gage portion 74can be selected based on various factors. With an OD of about 0.50inches, for example, the ID can be selected from a range of from about0.41 inches (10.414 mm) to about 0.44 inches (11.176 mm).

The deleterious effects of the molten salt on the load bearingproperties of the metal of the gage portion 74 increase as thesalt-metal contact area (e.g., the inner surface area of the gageportion) increases, and the deleterious effects of the molten salt onthe load bearing properties of the metal of the gage portion decrease asthe total volume of metal in the gage portion increases. Thus, themolten salt's effect on the load bearing properties of the metal of thegage portion 74 is generally proportional to the ratio of Ai/V, where Aiis the salt-metal contact area in the gage portion and V is the volumeof metal in the gage portion. For a cylindrical gage portion, the ratioAi/V can be equal to 4·ID/(OD²−ID²). Increasing the inner diameter ID,and thereby decreasing the thickness T, has the effect of increasing Aiand decreasing V, thereby increasing the Ai/V ratio and increasing theeffect of the salt on the strength of the metal. Increasing the Ai/Vratio is generally desirable for providing more significant observableeffects of the salt on the metal. For example, an Ai/V ratio (in unit ofsquare inches/cubic inches) greater than 2, greater than 10, and/orgreater than 20 can be achieved. The dimension ranges provided aboveresult in an Ai/V ratio (in unit of square inches/cubic inches) of fromabout 20 to about 32.

However, if the wall thickness T is too small, the total load bearingcapacity of the gage portion 74 can become so small that loading errortolerances become too significant. Furthermore, manufacturing tolerancelimitations can restrict how small the wall thickness T can be whileproviding accurate testing conditions. For example, with an OD of about0.50 inches, a minimum wall thickness T can be about 0.06 inches.

It can be desirable to compare the performance of the disclosed testspecimen to the performance of a solid test specimen that has its outersurface exposed to atmospheric air, and is not exposed to salt, duringtesting. Such a comparison can help determine how significant theeffects of the molten salt are on the test specimen material. However,for a more accurate comparison, it can be desirable to have the solidspecimen and the molten salt specimen have the same or similar Ai/Vratios. In a solid specimen, Ai is the outer air-metal contact area andV is the total volume of metal. In the exemplary molten salt testspecimens described above with an OD of about 0.5 inches and an IDselected from a range of about 0.41 inches to about 0.44 inches, theresulting Ai/V ratio range (in unit of square inches/cubic inches) offrom about 20 to about 32 can be about equal to the Ai/V ratio range fora comparative solid test specimen having an OD of from about 0.125inches (3.175 mm) to about 0.2 inches (5.08 mm), which corresponds tothe dimensions of common solid specimen types for which ample test datais available for comparison to data collected from the herein describedmolten salt environment testing techniques.

The inner void 76 of the test specimen 4 contains the molten salt duringtesting such that the entire gage length L₂ of the void is filled withthe molten salt and the upper level of the molten salt is within theupper connector 70. This ensures that the entire inner surface area ofthe gage portion 74 remain in contact with the molten salt duringtesting. However, it can be challenging to get the salt into the void 76and seal the salt within the void while minimizing the amount ofcontaminants that become trapped in the void 76 as well. Most applicablesalts are in solid phase below about 400° C., so they are in the solidphase when the salt is placed into the void 76 at room temperature, andthus it can be difficult to avoid trapping air, moisture, and/or othercontaminants in the void 76 along with the solid salts. For example, thesalts can easily oxidize or adsorb oxygen and moisture if exposed to airor moisture, in either the solid or liquid phase. Trapped oxygen,moisture and other oxidizers within the specimen void can react with thetest specimen material, especially at high temperatures.

To minimize contamination and oxidation of the salt, the salt can beinitially placed into a mold in a granular or chunk form and then heatedand melted in an very low oxygen and moisture environment to minimizeimpurities such as oxygen, moisture, and other gases from the surface ofthe salt particles and within the voids between the salt particles andto form a solid ingot having dimensions to completely fill the voidwithin the test specimen. The heated formation of the ingot can removemoisture, air, and other contaminants from the salt and produce a saltingot with substantially reduced contaminants; and because the ingotfills the void 76, the air and moisture in the void is forced out whenthe ingot is inserted into the void and sealed closed, such thatreactions of the internal surface of the test specimen with contaminantssuch as oxygen and moisture is minimized.

The salt ingot preparation procedure can be performed in vacuum or otherenvironments of very low oxygen and very low moisture content, whichminimizes exposure of the salt to air and moisture. Salt, particularlyin a powdered form, readily adsorbs oxygen and moisture from the air,which can cause corrosion and/or oxidation when the salt is placed inthe specimen and heated. Preparing the salt in a vacuum addresses thatproblem.

The use of a solid salt ingot can also help to ensure that the gageportion of the test specimen 4 is completely filled with molten saltduring testing. When the solid ingot melts, its upper surface level doesnot change significantly from the solid phase to the liquid phasebecause there is minimal void space within the ingot and between theingot and the internal surface of the test specimen. However, when apowdered salt is placed directly into the specimen, the upper surfacelevel of the salt can drop significantly when the salt melts and the gastrapped in the voids of the powder vaporizes and rises to the top of thesalt. Thus, a sufficient volume of powder must be added into the testspecimen so that the upper surface level remains above the gage portionafter the salt melts.

Exemplary Salt Ingot Preparation System

FIGS. 7 and 8 illustrate an exemplary salt ingot preparation system 100for creating a salt ingot for use in the herein described testingsystems. FIG. 7 shows certain components of the salt ingot preparationsystem 100 individually, and FIG. 8 shows the system 100 assembled.

As shown in FIGS. 7 and 8, the system 100 can comprise an ingot mold 102(e.g., made of graphite), a funnel 104 (e.g., made of graphite), a moldhousing 106, a furnace 108, a vacuum chamber 110, a thermocoupleattachment 112, and a quick release seal 114 for the mold housing. Thefunnel 104 is configured to be placed on top of the mold 102 to allowsolid salt to be poured into the mold. The mold 102 and funnel 104 areconfigured to be positioned within the mold housing 106. As illustrated,the mold housing 106 is missing a clamp ring for the quick release seal114. The quick release seal 114 as illustrated is also missing an axialspring that is positioned along the shaft of the quick release seal. Thequick release seal 114 is configured to be positioned over the moldhousing 106 and seal the mold and funnel within the mold housing. Thefurnace 108 is configured to be positioned within the vacuum chamber 110and the mold housing 106 is configured to be positioned within thefurnace. The thermocouple attachment is configured to be attached to aport 116 of the vacuum chamber 110, as shown in FIG. 8. The vacuumchamber 110 can also include an electrical power port 118 for connectingpower to the furnace 108 (FIG. 8), a vacuum port 120, a gas port 134,and an upper member 136 (with an optional optical window) that seals offthe top of the chamber 110 (FIG. 8).

FIG. 8 shows the assembled vacuum chamber 110 in a sealed state,optionally with one or more of the furnace 108, mold housing 106, moldhousing clamp ring, funnel 104, mold 102, and the salt sealed within thechamber. An electrical power supply is attached to the electrical powerport 118 to power the furnace, the thermocouple attachment 112 isattached to the port 116 to monitor the temperature of the furnacewithin the chamber 110, a valved gas line is attached to the gas port134, and a vacuum pump 128 is attached to the vacuum port 120. A lowvacuum gauge 122 and a high vacuum gauge 124 can also be attached to thevacuum line 120 to monitor the vacuum level. A vacuum valve 126 ispositioned between the vacuum pump 128 and the vacuum line 120 into thechamber 110. The vacuum pump 128 can comprise a turbomolecular vacuumpump, for example. In some embodiments, an outlet line 130 from thevacuum pump 128 is coupled to a roughing vacuum pump (not shown), suchas for a turbomolecular vacuum pump. The gas line attached to the gasport 134 can be coupled to a supply of a process gas, such as Argon-4%Hydrogen (Ar-4% H2), which is pumped into the vacuum chamber to replacethe ambient air and minimize contamination and corrosion and/oroxidation.

In an alternative embodiment, the furnace or other heat source can belocated outside of the vacuum chamber. As such, the vacuum chamber canhave a smaller volume. The smaller volume within the vacuum chamber canreduce the time needed to achieve the desired vacuum level within thechamber and/or can increase the time needed to achieve the meltingtemperature.

Exemplary Salt Ingot Preparation Procedures

The following describes an exemplary procedure for preparing a saltingot and installing the salt ingot into a test specimen for use in thetesting systems described herein.

In an initial “bake-out” phase, oxygen, moisture and other impuritiesare removed from the equipment used to make the salt ingot. Oxygen andmoisture in combination with molten salts, such as molten fluoride saltscan cause increased corrosion of alloys. The bake-out phase can includeall or some of the following steps:

1. Install funnel, mold, and mold housing into vacuum chamber centeredwithin furnace.

2. Attach connections and flanges on vacuum chamber and seal vacuumchamber.

3. Pull high vacuum on vacuum chamber.

4. Back fill vacuum chamber with Ar-4% H2.

5. Optionally repeat steps 3 and 4 as needed. Target vacuum is greaterthan 10⁻⁶ torr.

6. In high vacuum, heat with furnace to 850° C.

7. Hold at temperature over 48 hours for first use, 6 hours forsubsequent batches.

8. Cool down in high vacuum.

In a subsequent “salt fill” stage, the salt is placed in the mold andsealed without exposing the salt to air or moisture. The salt fill stagecan include all or some of the following steps:

9. Start Ar-4% H2 flow.

10. With Ar-4% H2 flowing, open vacuum chamber.

11. With Ar-4% H2 flowing, quickly place a mold housing seal onto moldhousing and clamp closed.

12. With Ar-4% H2 flowing, remove mold housing from vacuum chamber.

13. With Ar-4% H2 flowing, reseal vacuum chamber.

14. Transport sealed mold housing to a glove box with very low oxygenand very low moisture content, such as via an anti-chamber. Steps 9-14allow for mold and funnel to be transported into the glove box withminimal exposure to air and moisture.

15. In glove box, weigh out the appropriate amount of previouslypurified salt (e.g., as irregularly shaped chunks). In the case offluoride salts, purification steps include removal of adsorbed oxygenand moisture, hydrofluorination of the melted salt to remove additionalimpurities such as oxides and sulfur compounds, flushing of the processgases, and filtration of the melted salt. Post-purification, the saltcan be maintained in very low oxygen and very low moisture environmentsto avoid contamination of the salt with oxygen and moisture.

16. In glove box, remove mold housing seal and load salt into funnel andmold.

17. In glove box, replace mold housing seal and clamp closed. Steps15-17 are performed in a very low oxygen and very low moistureenvironment of a glove box to load the appropriate quantity of salt thatwill fill the internal void of the test specimen to a level above thegage length, and prepare for transport back to the vacuum chamber.

In a subsequent “ingot fabrication” phase, the salt is melted andre-solidified within the mold to remove impurities and voids within thesalt. The ingot fabrication phase can include all or some of thefollowing steps:

18. Transport sealed mold housing from glove box to vacuum chamber.

19. With Ar-4% H2 flowing, open vacuum chamber.

20. With Ar-4% H2 flowing, place mold housing into furnace.

21. With Ar-4% H2 flowing, remove mold housing seal and gasket.

22. With Ar-4% H2 flowing, place thermocouple into salt.

23. With Ar-4% H2 flowing, reseal vacuum chamber.

24. Pull high vacuum on chamber.

25. Back fill with Ar-4% H2. Repeat steps 24 and 25 as needed.

26. In flowing Ar-4% H2, heat system, e.g. to 600° C., until salt melts.

27. In flowing Ar-4% H2, cool down to room temperature. This processresults in the fabricated ingot of highly purified salt with minimalre-contamination with oxygen and moisture.

In a subsequent “ingot installation” phase, the salt ingot fabricated inthe ingot fabrication phase is placed into and sealed within a testspecimen while minimizing exposure of the ingot. The ingot installationphase can include all or some of the following steps:

28. With Ar-4% H2 flowing, open vacuum chamber.

29. With Ar-4% H2 flowing, quickly place mold housing seal onto moldhousing and clamp closed.

30. With Ar-4% H2 flowing, remove sealed mold housing and reseal vacuumchamber.

31. Transport sealed mold housing to glove box with low moisture and lowoxygen atmosphere (e.g., in weld area). Steps 28-31 allow ingot, moldand funnel to be transported without exposure to air and moisture.

32. In glove box, remove mold housing seal and remove salt ingot frommold.

33. In glove box, load salt ingot into test specimen void.

34. In glove box, add end plug to test specimen to seal ingot withinspecimen.

35. In glove box, weld end plug in place. This allows the specimen to betested without exposing the salt to oxygen or moisture.

The above described method for preparing the salt ingot can be used forpurified salt that cannot be readily reduced to sufficiently smallparticles. For salt that when purified can be readily reduced tosufficiently small particles, an alternative method can be used whereinthe required quantity of the purified salt is loaded directly into thespecimen in a very low oxygen, very low moisture glove box.

Exemplary Testing Procedures

The following describes an exemplary procedure for preparing anexemplary stress-rupture testing system and test specimen, such asdescribed herein, and testing the test specimen with a salt ingot sealedwithin the specimen, as described herein. Preparing and using thetesting system can include all or some of the following steps:

1. Apply boron nitride to threads on upper and lower ends of the testspecimen to prevent seizing up after long exposure to high testingtemperature under load, assemble the basin ring to the specimen belowthe gage portion, and then install the specimen into the load train.

2. Install three thermocouples at the top, middle, and bottom of thespecimen gage portion.

3. Install extensometery components to the specimen and the basin ring.

4. Install a salt leak indicator cable, such as along the upper pullrod, and fix an indicator tip of the cable to the basin ring.

5. Install the thermal break assembly on bottom of the load train.

6. Test thermocouples to make sure they all work properly.

7. Clean the load train, such as with alcohol, to remove impurities thatmay generate contaminating gases at high testing temperature inside theenvironmental chamber. Place insulation cloth around the pull rod abovethe thermal break adaptor, and insert the load train into theenvironmental vessel.

8. Install the lower cross chamber right flange with the cooling coilaround the thermal break lower pull rod inside the lower cross chamber.

9. Install the load cell onto the lower pull rod below the bottom of thethermal break.

10. Connect the load cell signal leads to its inlet cable on the leftflange of the lower cross chamber and then install the left flange tothe lower cross chamber.

11. Screw the bottom pull rod into the load cell while holding the loadcell.

12. Install LVDTs to the extensometer.

13. Attach the bottom end flange to the lower cross chamber andorientate so the inert gas line easily connects to the bottom endflange.

14. Tighten down all flanges.

15. Connect all gas lines.

16. Connect outside load cell cable to its inlet adaptor on the leftflange.

17. Connect LVDT cables to data acquisition system.

18. Connect the three thermocouples and the salt leak indicator to thetop of the vessel.

19. Connect water cooling lines on top pull rod, bottom pull rod, andright flange.

20. Attach load train to loading source, such as a creep machine.

21. Flush the vessel with purified argon flowing in from the bottom andout from the top. Because argon is heavier than oxygen and nitrogen,which are the major elements in air, the argon atmosphere that buildsfrom the bottom gradually pushes the air up and out through the top gasline outlet.

22. The argon flow rate is set to fill the vessel in approximately 45minutes. After approximately 180 minutes, which allows argon 3-4 timesof the vessel volume to flush through the vessel, the system is readyfor temperature increase.

23. Turn on the salt leak indicator.

24. Turn on the recirculation water system to start the cooling waterflow.

25. Start the furnace and slowly raise the furnace temperature to theprescribed testing temperature.

26. When the prescribed temperature is stabilized, reset dial gage, addprescribed testing weight to load train, and start the timer. Teststarts.

27. Monitor specimen during testing until specimen fails, such as byrupturing at the gage portion.

28. Remove load, decrease temperature, and remove argon and collectsalt.

Exemplary Creep Testing Assembly with Extensometry

FIGS. 9-12 show an exemplary creep testing system 202 for testing a testspecimen 204 in an environment simulating a molten salt nuclear reactoror molten salt heat exchanger. FIG. 9 shows an upper portions of thesystem 202, FIG. 10 shows an intermediate portion of the system andFIGS. 11 and 12 show two orthogonal view of a lower portion of thesystem. The system 202 is similar to the system 2 described herein, withsome significant differences. In the system 202, the system measures thechange in the length of the test specimen as a function of time and/ortension load applied. To accomplish this, the system 202 includesextensometry components that measure the change in length of the testspecimen. The extensometry components can include one or more axiallytranslating members (see, e.g., tubes 294 in FIGS. 10-12) coupled to asensor, such as a linear variable differential transformer (LVDT), thatdetects the relative axial motion between the axially translating memberas a reference stationary position.

The test specimen 204 (FIG. 10) can have features that are similar astest specimen 4 disclosed above, with a longer shoulder section toaccommodate addition of a ridge ring that provides attachment for theextensometry upper retainer 290. The specimen 204 is shown held in asubstantially vertical orientation in a load train of the testing system202 and positioned within an elongated vessel 206. In other embodiments,the load train can be oriented non-vertically, such as horizontally orat other angles from vertical. The illustrated embodiment is orientedvertically to utilize gravitational forces to apply constant tension tothe test specimen and ensure that the molten salt fills the gage portionof the specimen during testing. However, in other embodiments othermechanisms can be used to apply tension to the test specimen while thespecimen is at a non-vertical orientation, while maintaining sufficientsurface contact of the molten salt against the specimen material.

The vessel 206 is used to provide a controlled environment around thetest specimen and the load train during testing. For example, the vessel206 can be filled with an inert gas, such as argon, during testing toprotect the load train components and the test specimen from oxidizingor otherwise reacting to the ambient environment in the vessel at hightemperatures. For example, in some embodiments, load train componentscan comprise titanium-zirconium-molybdenum alloy (TZM), which can besusceptible to reaction with oxygen and other elements in atmosphericair at high testing temperatures. The vessel 206 can also be constructedof a non-reactive material, such as alumina. The vessel 206 can besupported independently from the load train, such as by mountingbrackets 226 and 227. The system 202 also includes a heating mechanism,such as the furnace 208, adjacent to the vessel 206 for maintaining thetest specimen and salts at a desired temperature during testing.

The load train can comprise an aligned series of components that holdthe test specimen 204 within the testing system 202 and apply desiredloads to the test specimen during testing. The load train can compriseupper components that couple the test specimen 204 to a stationary uppermounting location and lower components that couple the test specimen toa loading source (not shown) below the system 202. When a load isapplied by the load train during testing, a gage portion 274 of the testspecimen 204, which is exposed to the molten salt, is stressed todetermine the test specimen material's ability to withstand theenvironmental rigors of a molten salt nuclear reactor. The data can thenbe used to characterize the material being tested.

The upper part of the load train (see FIG. 9) can include couplings 210,214 and pull rods 212, 216 that couple an upper end of the test specimen204 to a stationary upper support (not shown) via a threaded upper end222 of the pull rod 216. The upper portion of the load train extendsthrough an opening at the upper end 218 of the vessel 26, which can besealed, such as with a bellows-type seal 220, to prevent inert gas fromescaping or atmospheric gas from entering the vessel.

The system 202 can include thermal shields 230, 236 mounted on the loadtrain above and below the test specimen 24, as shown in FIG. 10. Theshields 230, 236 reduce convection and radiation heat loss from thetesting zone where the specimen gage section locates to facilitatemaintaining a uniform temperature gradient within the testing zone andminimizing heat outside the testing zone. The shields 230, 236 can bespaced slightly from the inner walls of the vessel 206 to avoidfrictional forces on the load train that can disrupt the testingprocedure.

As shown in FIGS. 11 and 12, the lower part of the load train couplesthe lower end of the test specimen to the loading source. A lower end ofthe load train extends through a seal assembly 256 at the lower end ofthe lower chamber 250 of the vessel 206 and includes a threaded lowerend 266 that is configured to attach to the loading source below thetesting system 202. The loading source can be a dead weight hanging fromthe bottom of the load train, an actuator, or any other loadingmechanisms. The lower part of the load train can include a thermal breakbetween the components that are coupled to the lower end of the testspecimen 204 within the furnace 208 (e.g., coupling 232 and pull rod234), and the lower components of the load train (e.g., lower pull rod244) that are coupled to the load source. The thermal breaksignificantly limits heat conduction downwardly through the load train,which protects temperature sensitive components below the thermal break.

For example, a load cell 254 that is vulnerable to high temperatures canbe attached to, or be inserted between upper and lower portions of, thelower pull rod 244 below the thermal break for measuring the amount ofload applied to the test specimen. The load cell 254 can be electricallycoupled to data collection systems or closed-loop load control systemoutside the system 202 via connection 255. Placement of the load cell254 within the vessel 206 allows for more accurate measurement of theload that is applied to the test specimen 4 as it accounts forfrictional influence between the lower pull rod 244 and the sealassembly 256 of the vessel or elastic force when a bellows-type seal isused. Placement of the load cell 254 within the vessel 206 is enabled byuse of the thermal break. The load cell 254 can be threadably insertedbetween an upper portion of the lower pull rod 244 and a lower portionof the lower pull rod, such that the load cell can measure the totalamount of force that is transmitted along the lower pull rod 244.

The thermal break (see FIGS. 11 and 12) provides a break in the thermalconduction pathway downward through the load train. The pull rods (e.g.,234, 244) and couplings (e.g., 232) of the load train can comprise ametal material that maintains high strength at very high testingtemperatures, such as TZM. Unlike other common metals, TZM load traincomponents remain very strong and provide sufficient strength to thespecimen grips and other load train connections during the hightemperature conditions during testing. However, TZM and otherhigh-temperature-resistant metals can also be good thermal conductors.The thermal break includes thermally insulating material, such asceramic materials, such that the pull rod 234 above the thermal break isthermally insulated from the lower pull rod 244 below the thermal break.

Thermal break can include a thermal break adaptor 238, a thermal breakfixture 240, and an insulating spacer 242. The adaptor 238 and fixture240 can comprise metal materials that are strong in tension, like theTZM pull rods 234 and 244. The adaptor 238 can be threadably attached tothe lower end of the pull rod 234, and the fixture 240 can be threadablyattached around the adaptor 238. The insulating spacer 242 can comprisea ceramic disk, for example, that rests on a lower ledge 249 of thefixture 240 and supports an upper rim or head 246 of the pull rod 244.The pull rod 244 passes through a lower opening 248 in the fixture 240without contacting the fixture to prevent direct thermal conduction fromthe fixture 240 to the pull rod 244. The upper head 246 of the pull rod244 is flared radially such that the upper head forms a lower contactsurface that rests on top of the spacer 242. The spacer 242 can fittightly around pull rod 244 under the head 246 to restrain the pull rod244 from moving radially relative to the spacer, and the spacer 242 canalso fit tightly within the fixture 240 to restrain the spacer frommoving radially relative to the fixture. A ceramic material of thespacer 242 can provide high strength in compression between the pull rodhead 246 and the lower ledge 249 of the fixture 240, while alsoproviding thermal insulation. Furthermore, the upper surface of the pullrod head 246 is spaced from the lower surfaces of the pull rod 234 andadapter 238, and spaced radially from the inner surfaces of the fixture240 to minimize thermal conduction. The adapter 238 can be removed fromthe fixture 240 to provide access for inserting the spacer 242 and thepull rod 244 into the fixture. The fixture 240 can comprise a generallycylindrical or tubular configuration or can have other suitableconfigurations.

The vessel 206 can comprise a lower chamber 250 below the furnace, suchas a cross-chamber having lateral access ports. The lower chamber caninclude a cooling system 252 and related ports 253 for cooling the lowerend of the load train, such as a fluid that flows through a coil wrappedaround the pull rod 244 below the thermal break. The load cell 254 canbe positioned below the coil of the cooling system 252 such that theload cell is further protected from thermal damage. In addition to thethermal break and the cooling system 252, the various components of thetesting system 202 can include additional cooling systems to dissipateheat. For example, the vessel 206 can include fluid ports 229 and thelower chamber 250 can also include fluid ports for circulating a coolingfluid, such as water. In addition, the upper pull rod 216 and lower pullrod 244 can include fluid-cooled adapters 224 and 264, respectively, forcirculating cooling fluid around and/or through the pull rods. In someembodiments, the pull rods can include one or more hollow passagewaysextending through the pull rods for circulating cooling fluid.

As shown in FIGS. 11 and 12, the lower end of the vessel includes a sealassembly 256 that allows the pull rod 244 to pass through the lower endof the vessel with minimal frictional resistance to relative verticalmotion. A small gap 257 is allowed between the pull rod 244 and theinner walls of an opening passing through the seal assembly 256 suchthat the pull rod 244 does not contact the inner walls of the sealassembly. A bushing 258 and/or a ring 260 can be positioned around thepull rod 244 and can fit securely within a lower recess 262 of the sealassembly below the gap 257 to keep the pull rod centered in the loweropening and maintain the small gap 257, while allowing the pull rod toslide vertically relative to the vessel with minimal friction. Thebushing 258 and/or ring 260 can be made of a material that has a verylow coefficient of friction with the pull rod 244. The bushing 258and/or ring 260 can include one or more vertical holes, such as drilledholes, to further facilitate the escape of inert gas through the gap257.

The vessel 206 can include one or more process gas inlets, such as inlet268 in the lower end seal assembly 256 and/or an inlet in the upper end218 of the vessel (not shown), which allow an inert process gas, such asargon, to be fed into the vessel. The process gas can escape through thegap 257 and/or through other outlets, such as an upper outlet at the topend of the vessel, while additional process gas can be fed into thevessel through one or more inlets, such as the inlet 268, to maintain apositive flow out of the vessel 206, which inhibits unwanted externalgases from entering the vessel. During set up of the system, atmosphericair captured within the vessel 206 can initially be purged out throughan upper gas outlet near the top end of the vessel (since the processgas is heavier than air) by feeding in the process gas through one ormore inlets, such as lower inlet 268, for a sufficient period of time.Other ports can also be provided in the vessel 206, such as ports 219for thermocouples that attach to the test specimen, a salt leakindicator attached to a basin ring 278, and ports for other sensors andwiring.

The test specimen 204 and the basin ring 278 can be attached around alower threaded portion 282 of the test specimen 204 for catching moltensalt that escapes from the test specimen when it ruptures or leaksduring testing. The test specimen 204 includes an upper connector 270, alower connector 272, and a gage portion 274 between the upper and lowerconnectors. The connectors 270 and 272 are threaded to attach to upperand lower couplings 210 and 232, respectively, of the load train. Thecouplings 210 and 232 are also referred to as grips. The threaded upperand lower connectors 270, 272 are sized such that they are stronger intension than the gage portion 274, such that stress-rupture failureoccurs first in the gage portion. The test specimen 204 includes aninner void 276 that extends through the gage section and the upperconnector 270. The void 276 is open at the upper end of the upperconnector for loading the salt and then closed off prior to testing(e.g., immediately after loading the salt) to ensure containment of themolten salt during testing and to protect the salt from contamination.The void 276 is configured to be welded closed, for example, to containthe salt in its corrosive liquid phase during testing at hightemperatures.

The system 202 can further include extensometry components within thevessel 206 coupled to the load train, such as an upper retainer 290, oneor more axially extending rods 292, one or more axially extending tubes294, a lower retainer 296, one or more LVDT holders 297, one or moreLVDTs 298, and wiring components 299 associated with the LVDT (see FIGS.10-12). The upper retainer 290 can be coupled to stationary locationabove the test specimen, such as an upper portion of the test specimen(e.g., upper connector 270) or to an upper portion of the load train(e.g., coupling 210 or pull rod 212). The upper retainer 290 can supportthe upper ends of one or more rods 292 that extend down past the testspecimen, past the thermal break, to the LVDT devices 298 located in thelower chamber 250. Because the rods are fixed to the upper retainer 290,the rods can provide a stationary reference as the test specimenelongates under load. The rods 292 can pass through tubes 294 that arecoupled to the lower end of the test specimen or a lower part of theload train that moves downward as the test specimen elongates. As shownin FIG. 10, the tubes can pass through and optionally be fixed to thelower basin ring 278 and, as shown in FIG. 11, can pass through andoptionally be fixed to the lower retainer 296. The upper ends of thetubes 294 may be located at a vertical level high enough to avoid moltensalt entering the tops of the tubes when the test specimen ruptures.

As the test specimen elongates under load testing, the tubes 294 movedown over the stationary rods 292. The LVDT holders 297 can be coupledto the lower ends of the tubes 294 and move axially with the tubes. TheLVDT holders 297 retain the LVDTs 298, which are positioned around oralongside the lower ends of the rods 292, such that the LVDTs move downrelative to the rods as the test specimen elongates. The LVDTs 298detect the relative motion and convert it to a signal that istransmitted via wiring 299 that extends out of the lower chamber 250 toa computing, processing, recording, and/or transmitting device.

The lower chamber 250 can be configured to accommodate the extensometrycomponents in addition to the cooling components, load cell components,etc. For example, the lower chamber 250 can comprise a double-crosschamber, which can provide more space and access points to accommodatethe added extensometry components and wiring. FIG. 11 shows oneorthogonal cross-sectional view of the lower cross-chamber 250, withlateral ports for wiring 299, and FIG. 12 shows another orthogonalcross-sectional view of the lower cross-chamber 250, with additionallateral ports for the cooling elements 253 and the load cell wiring 255.

More information regarding the disclosed technology and relatedtechnology can be found in U.S. Pat. No. 9,291,537, issued on Mar. 22,2016, the entire contents of which is incorporated by reference herein.

Unless otherwise noted, technical terms are used herein according toconventional usage. In order to facilitate review of the variousembodiments of the disclosure, the following explanation of terms isprovided.

The singular terms “a”, “an”, and “the” include plural referents unlesscontext clearly indicates otherwise. The term “comprises” means“includes without limitation.” The term “coupled” means directly orindirectly linked and does not exclude intermediate elements between thecoupled elements. The term “and/or” means any one or more of theelements listed. For example, the term “A and/or B” means “A”, “B” or “Aand B.”

Although methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present technology,only certain suitable methods and materials are described herein. Incase of conflict, the present specification, including terms, willcontrol. For purposes of this description, certain aspects, advantages,and novel features of the embodiments of this disclosure are described.The disclosed methods, apparatuses, and systems should not be construedas limiting in any way. Instead, the present disclosure is directedtoward all novel and nonobvious features and aspects of the variousdisclosed embodiments, alone and in various combinations andsub-combinations with one another. The methods, apparatuses, and systemsare not limited to any specific aspect or feature or combinationthereof, nor do the disclosed embodiments require that any one or morespecific advantages be present or problems be solved.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language. Forexample, operations described sequentially may in some cases berearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed methods can be used in conjunction with other methods.

Unless otherwise indicated, all numbers expressing properties, sizes,percentages, measurements, distances, ratios, and so forth, as used inthe specification or claims are to be understood as being modified bythe term “about.” Accordingly, unless otherwise indicated, implicitly orexplicitly, the numerical parameters set forth are approximations thatmay depend on the desired properties sought and/or limits of detectionunder standard test conditions/methods. When directly and explicitlydistinguishing embodiments from discussed prior art, numbers are notapproximations unless the word “about” is recited.

In view of the many possible embodiments to which the principles of thedisclosed technology may be applied, it should be recognized that theillustrated embodiments are only examples and should not be taken aslimiting the scope of the disclosure. Rather, the scope of thedisclosure is at least as broad as the following claims and theirequivalents. I therefore claim all that comes within the scope of thefollowing claims.

1. A system for creep testing, fatigue testing, creep-fatigue testing,or relaxation testing of materials in a high-temperature molten saltenvironment, the system comprising: a vessel having an upper end and alower end, the vessel comprising one or more gas ports for maintaining acontrolled inert gas environment within the vessel; a first pull rodportion positioned within the vessel and extending downwardly from orthrough the upper end of the vessel, the first pull rod portion having afirst specimen grip at a lower end of the first pull rod portion, thefirst specimen grip adapted to grip an upper end of a test specimenhaving a tubular gage portion for containing a salt; a second pull rodportion positioned within the vessel below the first pull rod portion,the second pull rod having a second specimen grip at an upper end of thesecond pull rod portion, the second specimen grip adapted to grip alower end of the test specimen; and an extensometry system configured tomeasure elongation of a test specimen while a load is applied.
 2. Thesystem of claim 1, wherein the extensometry system comprises a fixedaxial member coupled to the upper shoulder portion of the test specimenand an axially translating member coupled to the lower shoulder portionof the test specimen, wherein the axially translating member moves downrelative to the fixed axial member as the test specimen elongates. 3.The system of claim 2, wherein the extensometry system comprises atleast one LVDT that measures relative motion between the fixed axialmember and the axially translating member as the test specimenelongates.
 4. The system of claim 2, wherein the axially translatingmember comprises a tube and the fixed axial member comprises a rod thatextends through the tube.
 5. The system of claim 1, wherein theextensometry system is contained within the vessel.
 6. The system ofclaim 1, further comprising a thermal break positioned within the vesseland coupled to a lower end of the second pull rod portion, the thermalbreak comprising a fixture coupled to a lower end of the second pull rodportion and a thermally insulating spacer supported by the fixture belowthe second pull rod portion.
 7. The system of claim 6, furthercomprising a third pull rod portion having an upper end spaced below thelower end of the second pull rod portion and spaced within the thermalbreak fixture, the upper end of the third pull rod portion beingsupported by the thermally insulating spacer such that the third pullrod portion is thermally decoupled from the second pull rod portion bythe thermally insulating spacer, the third pull rod portion having alower end that extends through a lower end of the vessel and is adaptedto be coupled to a loading source for applying a load to the testspecimen via the second and third pull rod portions and the thermalbreak.
 8. The system of claim 6, wherein the extensometry systemincludes a portion located above the thermal break and a portion locatedbelow the thermal break.
 9. The system of claim 8, wherein theextensometry system comprises at least one LVDT that measures relativemotion between the fixed axial member and the axially translating memberas the test specimen elongates, and the LVDT is located below thethermal break.
 10. The system of claim 7, further comprising a load cellcoupled to the third pull rod portion within the vessel below thethermal break, the load cell being thermally protected by the thermalbreak and configured to measure the load applied to the test specimenvia the third pull rod portion.
 11. The system of claim 1, furthercomprising a furnace positioned around at least a portion of the vesselfor maintaining the test specimen at a desired temperature that issufficient to cause a salt within the test specimen to be in a liquidphase.
 12. The system of claim 7, wherein the thermal break fixturecomprises a metallic tubular body having an upper end secured to thesecond pull rod portion and a lower end forming an inner ledge thatsupports a lower surface of the thermally insulating spacer.
 13. Thesystem of claim 12, wherein the thermally insulating spacer comprises aceramic disk and the upper end of the third pull rod portion comprises aflared head that contacts an upper surface of the ceramic disk and isspaced apart from the fixture and the second pull rod portion.
 14. Thesystem of claim 1, wherein the vessel comprises a lower opening throughwhich the load train extends, there being a gap between the load trainand the lower opening such that friction between the load train and theopening is minimized or eliminated.
 15. The system of claim 7, furthercomprising a cooling coil coupled to the load train within the vesselbelow the thermal break.
 16. The system of claim 1, wherein the systemis capable of applying a stress load to the test specimen while the testspecimen is maintained at a temperature greater than 700° C.
 17. Thesystem of claim 1, wherein the salt comprises 2⁷LiF—BeF₂, KF—ZrF₄, otherfluoride or chloride containing salts, and the like.
 18. A method oftesting a selected material in a high-temperature molten saltenvironment, the method comprising: mounting a test specimen of aselected material in a load train within a vessel of a creep testingsystem, a solid salt being sealed within an inner void of the testspecimen; filling the vessel with an inert gas; heating the testspecimen, while mounted in the load train within the vessel filled withinert gas, such that the salt melts within the void and a resultingmolten salt contacts an inner surface of a gage portion of the testspecimen; applying a load to the gage portion of the test specimen whilethe test specimen is mounted in the load train within the vessel filledwith inert gas and the salt is molten; and measuring a change in axiallength of the gage portion of the test specimen while under the appliedload.
 19. The method of claim 18, wherein the measuring a change inaxial length of the gage portion comprises detecting relative axialmotion between an axially translating member fixed relative to a lowerend of the test specimen and a fixed axial member fixed relative to anupper end of the test specimen.
 20. The method of claim 18, wherein themeasuring a change in axial length of the gage portion comprisesdetecting relative axial motion using at least one LVDT.
 21. The methodof claim 18, wherein the method further comprises continuously keepingadequate inert gas pressure inside the vessel while the load is appliedand preventing air ingress into the vessel through a gap between theload train and a lower end of the vessel.
 22. The method of claim 18,wherein heating the test specimen comprising heating the test specimento a temperature that is at least 100° C. greater than the meltingtemperature of the salt.
 23. The method of claim 18, wherein furthercomprising measuring the applied load at least in part by a load cellmounted within the load train positioned within the vessel.
 24. Themethod of claim 18, further comprising placing the solid salt within theinner void of a test specimen by: placing a mold within a vacuumchamber; creating an inert gas environment within the vacuum chamberaround the mold; heating the mold within the vacuum chamber in the inertenvironment to remove impurities from the mold; after removingimpurities from the mold, placing a salt into the mold and closing themold in an inert environment; heating the mold to melt the salt andremove voids and impurities from the salt; cooling the mold to solidifythe salt into a salt ingot with impurities removed; and transferring thesalt ingot from the mold into a test specimen in an inert environment.25. The method of claim 24, wherein creating an inert gas environmentwithin the vacuum chamber comprises drawing a vacuum on the vacuumchamber and feeding an inert gas into the vacuum chamber to purgeambient air from the vacuum chamber.
 26. The method of claim 24, whereinthe method further comprises: placing a funnel and a mold housing withina vacuum chamber along with the mold; heating the mold, funnel, and moldhousing within the vacuum chamber in the inert environment to removeimpurities from the mold, funnel, and mold housing; transporting themold and funnel sealed within the mold housing from the vacuum chamberto a salt-filling chamber having an inert environment; using the funnelto place the salt into the mold in the salt-filling chamber; andtransporting the salt-filled mold sealed within the mold housing fromthe salt-filling chamber to a vacuum chamber.